Some of the symptoms to look out in case you suspect your dog of having depression are change in appetite, oversleeping, limp tail, loss of interest, avoidance.
Loneliness and neglect. Not paying enough attention to you dog and living alone at home most of the time, is one of the most frequent causes of depression. If you decide to adopt / buy a dog, treat him like family (because that's what he really is for you and that's what you are for your dog). Always pay enough attention to your dog so that he knows he is loved and cared.
Medical conditions. This is the number one cause of depression in dogs. That's why you should always take your dog to the vet as soon as you see any sign of depression in your dog. There are many medical conditions that can make your dog feel depressed and have low energy.
Family additions. Whether it is a human or a another animal, many dogs may feel threatened and may not like the idea of sharing your attention. Before adding to your family, try to get your dog used to the new situation. You can read more about preparing your dog for a new baby in the house here.
There are several ways on how to deal with dog depression, but the first thing you should do is taking them to the vet, and following his guidance. Your dogs might need medical cures, or therapy sessions depending on the causes of the depression.
But, don't forget to give to your dogs that one thing that cures all ... LOVE. Make them feel loved, cared, and give them all the attention they deserve so that you can prevent things from the start.